This invention relates to an improved focus-mask-type CPT (color television picture tube) and to a method for operating this improved CPT.
A commercial shadow-mask-type CPT comprises generally an avacuated envelope having therein a target comprising an array of phosphor elements of three different emission colors arranged in cyclic order, means for producing three convergent electron beams directed towards the target, and a color-selection structure including an apertured masking plate between the target and the beam-producing means. The masking plate shadows the target, and the differences in convergence angles permit the transmitted portions of each beam, or beamlets, to select and excite phosphor elements of the desired emission color.
At about the center of the color-selection structure, the masking plate of a commercial CPT intercepts all but about 18% of the beam currents; that is, the plate is said to have a transmission of about 18%. Thus, the area of the apertures of the plate is about 18% of the area of the mask. Since there are no focusing fields present, a corresponding portion of the target is excited by the beamlets of each electron beam.
Several methods have been suggested for increasing the transmission of the masking plate; that is, increasing the area of the apertures with respect to the area of the plate, without substantially increasing the excited portions of the target area. In one approach, the apertures are enlarged, and the beamlets are focused by magnetic or electric fields produced in the vicinity of each of the apertures. In a second approach, each aperture in the masking plate is enlarged and split into two adjacent windows by a conductor. The two beamlets passing through the windows of each aperture are deflected around the conductor towards one another, and both beamlets fall on substantially the same area of the target. In this second approach, the transmitted portions of the beams are also focused in one transverse direction and defocused in the orthogonal transverse direction.
One effort at such a combined deflection-and-focus color-selection means is described in West German Offenlegungschrift No. 2,814,391 published Oct. 19, 1978. That publication discloses a CPT having a target, as normally viewed, comprised of a mosaic of vertical phosphor stripes of three different emission colors arranged cyclically on triads (groups of three different stripes), means for producing three convergent horizontally in-line electron beams directed towards the target, and a color-selection structure located adjacent the target. The color-selection structure comprises a metal-masking plate having therein an array of substantially square apertures arranged in vertical columns and an array of narrow vertical conductors insulatingly spaced from the masking plate, with each conductor substantially centered over the apertures of one of the columns of apertures. Each aperture is also centered over a triad of phosphor stripes. Viewed from the electron-beam-producing means, the conductors divide each aperture into two essentially-equal horizontally-coadjacent windows. This prior color-selection structure has windows with a width-to-height ratio of about 0.46 and transmits about 44% or less of the electron beams.
When operating this latter device, the narrow vertical conductors are electrically positive with respect to the masking plate, so that the beamlets passing through each of the windows of the same aperture are deflected towards one another. Simultaneously, because of quadrupole-like focusing fields established in the windows, the beamlets are focused in the length direction of the phosphor stripes (compressed vertically) and defocused in the width direction of the phosphor stripes (stretched horizontally). The spacings and voltages are so chosen to form an electrostatic lens that also deflects the two beam parts to fall on the same phosphor stripe of the target. The convergence angle of the beam that produces the beamlet determines which stripe of the triad is selected. The voltage at the center of each window is higher than at the top and bottom thereof (resulting in vertical focusing) and is lower than at the left and right thereof (resulting in horizontal defocusing).
Careful analysis and experience with this color-selection structure have shown that the shapes of the deflected beamlets passing through each window, because they are elongated in the width (horizontal) direction and compressed in the length (vertical) direction, cause an overlapping of the beamlets onto the adjacent incorrect color phosphor stripe, or require a reduction in the widths of the windows, to assure adequate color purity in the image displayed on the target.